Treatment of melt for atomization technology

ABSTRACT

An improved method of manufacturing a powder metal material by water, gas, plasma, or rotating disk atomization is provided. The method includes adding at least one additive to a melted metal material before or during the atomization process. The at least one additive forms a protective gas atmosphere surrounding the melted metal material which is at least three times greater than the volume of melt to be treated. The protective atmosphere prevents introduction or re-introduction of contaminants, such as sulfur (S) and oxygen (O 2 ), into the material. The atomized particles produced include at least one of the following advantages: median circularity of at least 0.60, median roundness of at least 0.60, less internal pores, less internal oxides, and an increased sphericity of the microstructural phases and/or constituents.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates generally to powder metal materials, and methods of forming powder metal material by water or gas atomization or any other atomization process that requires the material to be atomized to go through the creation of a bath of liquid metal.

2. Related Art

Powder metal materials can be formed by various processes such as by water atomization, gas atomization, plasma atomization, or rotating disk. Common atomization processes include applying a fluid (water, gas, oil, or plasma) to a melted metal material to form a plurality of particles. During the water atomization process, the cooling rate of the melted metal is much faster than the cooling rate in gas atomization, which leads to the irregularly-shaped particles that are not generally desirable for metal injection molding, thermal spraying, additive manufacturing processes such as selective laser sintering, electron beam melting, three-dimensional printing and other manufacturing techniques wherein more spherical-shaped particles are preferred. Thus, the powder metal materials formed by water atomization are oftentimes used in typical press and sinter processes. Gas atomization is known to form particles having a more spherical shape. However, gas atomization is three to nine times more expensive than water atomization. Another common problem encountered in most atomized powders is the presence of internal porosities and internal oxides. These defects will negatively impact the mechanical properties of the parts made from the powders.

SUMMARY OF THE INVENTION

One aspect of the invention provides an improved method of manufacturing a powder metal material. The method includes adding at least one additive to a melted metal material that will form a protective gaseous atmosphere surrounding the melt. The protective atmosphere acts as a barrier to prevent impurities, such as sulfur (S) and/or oxygen (O₂), from entering or re-entering into the melted metal material; and atomizing the melted metal material after adding at least some of the additive(s) to produce a plurality of particles. The chemical nature of the selected additives(s) in relation with the chemical composition of the alloy to be atomized can produce at least one of these improvements: particles having a circularity median of at least 0.6 and a roundness median of at least 0.6, and/or less internal pores, and/or less internal oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is an optical photomicrograph of a comparative water atomized steel powder that contains about 1.3% C and 1.1% Si (FGP1210) without added magnesium screened at −200 mesh (74 microns and less) wherein the red arrows point to internal porosities;

FIG. 2 is an optical photomicrograph of a water atomized steel powder that contains about 1.4% C and 1.1% Si (FGP1210Mg, where “FGP” stands for Free Graphite Powder) with added magnesium screened at −200 mesh (74 microns and less) according to one example embodiment wherein the red arrows point to fewer and smaller internal porosities compared to those of FIG. 1;

FIG. 3 is a backscattered electron micrograph of a comparative water atomized cast iron powder that contains about 4.0% C and 2.3% Si (FGP4025) without added magnesium screened at −200 mesh (74 microns and less) wherein one red arrow points to one porosity;

FIG. 4 is a backscattered electron micrograph of a water atomized cast iron powder that contains about 4.1% C and 2.4% Si (FGP4025Mg) with added magnesium screened at −200 mesh (74 microns and less) according to one example embodiment wherein porosities were not observed compared to that of FIG. 3;

FIG. 5 is a backscattered electron micrograph of a comparative water atomized stainless steel powder (SS304) without added magnesium screened at −200 mesh (74 microns and less);

FIG. 6 is a backscattered electron micrograph of a water atomized stainless steel powder (SS304Mg) with added magnesium screened at −200 mesh (74 microns and less) according to one example embodiment;

FIG. 7 includes a table listing compositions subject to a water atomization process and evaluated;

FIG. 8 illustrates the circularity frequency distribution of powders having the FGP1210 compositions of FIG. 7 that were screened at −200 mesh (74 microns and less);

FIG. 9 illustrates the roundness frequency distribution of powders having the FGP1210 compositions of FIG. 7 that were screened at −200 mesh (74 microns and less);

FIG. 10 illustrates the circularity frequency distribution of powders having the FGP4025 compositions of FIG. 7 that were screened at −200 mesh (74 microns and less);

FIG. 11 illustrates the roundness frequency distribution of powders having the FGP4025 compositions of FIG. 7 that were screened at −200 mesh (74 microns and less);

FIG. 12 illustrates the circularity frequency distribution of powders having the SS304 compositions of FIG. 7 that were screened at −200 mesh (74 microns and less);

FIG. 13 illustrates the roundness frequency distribution of powders having the SS304 compositions of FIG. 7 that were screened at −200 mesh (74 microns and less);

FIG. 14 is a table illustrating numerical data for the circularity of the compositions listed in FIG. 7;

FIG. 15 is a table illustrating numerical data for the roundness of the compositions listed in FIG. 7.

FIG. 16 is a backscattered electron micrograph of a water atomized stainless steel powder (SS304) without added magnesium screened at −80/+200 mesh (between 177 and 74 microns), wherein the red arrows point to internal porosities;

FIG. 17 is a backscattered electron micrograph of a another water atomized stainless steel powder (SS304Mg) with added magnesium screened at −80/+200 mesh (between 177 and 74 microns), wherein one red arrow points to only one smaller internal porosity compared to those of FIG. 16;

FIG. 18 is a backscattered electron micrograph of a water atomized cast iron powder (FGP4025) without added magnesium in which many irregular primary graphite nodules precipitated on internal silicon oxides that were introduced in the melt during the pouring step of the atomization process;

FIG. 19 is a backscattered electron micrograph of another water atomized cast iron powder (FGP4025Mg) with added magnesium in which one spherical primary graphite nodule precipitated on a heterogeneous oxide nuclei that contains Mg during the atomization process;

FIG. 20 is a backscattered electron micrograph of a water atomized cast iron powder that contains about 4.0% C and 2.3% Si (FGP4025) without added magnesium wherein graphite nodules which grew in a solid state during a post heat treatment process are present;

FIG. 21 is a photomicrograph of another water atomized cast iron powder (FGP4025Mg) with added magnesium, according to an example embodiment, wherein more spherical graphite nodules compared to those presented in FIG. 20, which grew in the solid state during a post heat treatment process are present;

FIG. 22 illustrates the circularity frequency distribution of the graphite nodules in powders having the FGP4025 compositions of FIG. 7 after heat treatment;

FIG. 23 illustrates the roundness frequency distribution of the graphite nodules in powders having the FGP4025 compositions of FIG. 7 after heat treatment;

FIG. 24 is a table illustrating numerical data for the circularity of the graphite nodules that grew in the solid state for two powders for which the compositions are listed in FIG. 7;

FIG. 25 is a table illustrating numerical data for the roundness of the graphite nodules that grew in the solid state for two powders for which the compositions are listed in FIG. 7;

FIG. 26 is a graph showing the calculated total volume of gas that is obtained per 100 grams of melt as a function of the amount of additives in an example composition of FIG. 7;

FIG. 27 is a graph showing EDS spectra that were experimentally acquired on a polished pure iron surface before and after it was exposed to the atmosphere on top of the tundish during the atomization process of the powder that is described in FIG. 26;

FIG. 28 is a graph showing the calculated volume of gas generated by a sodium and potassium additive in aluminum at different temperatures (800 and 900 Celsius). The dashed line shows the inferior limit of gas;

FIG. 29 is a graph showing the calculated volume of gas generated by different additives in titanium at a temperature of 1800 Celsius. The dashed line shows the inferior limit of gas;

FIG. 30 is a graph showing the calculated volume of gas generated by different additives in cobalt at a temperature of 1600 Celsius. The dashed line shows the inferior limit of gas;

FIG. 31 is a graph showing the calculated volume of gas generated by different additives in chromium at a temperature of 2000 Celsius. The dashed line shows the inferior limit of gas;

FIG. 32 is a graph showing the calculated volume of gas generated by different additives in copper at a temperature of 1200 Celsius. The dashed line shows the inferior limit of gas;

FIG. 33 is a graph showing the calculated volume of gas generated by different additives in iron at a temperature of 1650 Celsius. The dashed line shows the inferior limit of gas;

FIG. 34 is a graph showing the calculated volume of gas generated by different additives in manganese at a temperature of 1400 Celsius. The dashed line shows the inferior limit of gas;

FIG. 35 is a graph showing the calculated volume of gas generated by different additives in nickel at a temperature of 1600 Celsius. The dashed line shows the inferior limit of gas;

FIG. 36 is a graph showing the calculated total volume of gas that is obtained per 100 grams of melt of a complex cobalt alloy at a temperature of 1600 Celsius as a function of the amount of additive (K and Li);

FIG. 37 is a table that presents the additives that will create a protective gas atmosphere for each chemical system (Al, Cu, Mn, Ni, Co, Fe, Ti, and Cr);

FIG. 38 is a table that presents the additives that will react with the dissolved sulfur for each chemical system (Al, Cu, Mn, Ni, Co, Fe, Ti, and Cr); and

FIG. 39 is a table that presents the additives that will react with the natural oxides of each chemical system (Al₂O₃ in Al, CuO in Cu, MnO₂ in Mn, NiO in Ni, CoO in Co, Fe₂O₃ in Fe, TiO₂ in Ti, and Cr₂O₃ in Cr).

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

One aspect of the invention includes an improved method of manufacturing a powder metal material by water or gas atomization or any other atomization process that requires that the material to be atomized goes through the creation of a bath of liquid metal such as plasma atomization or rotating disk atomization, by adding at least one additive to a melted metal material before and/or during the atomization process. The at least one additive forms a protective gas atmosphere surrounding the melted metal material which is at least three times greater than the volume of melt to be treated.

The at least one additive that is added to the melted materials will create a protective atmosphere that will act as a barrier to prevent impurities, such as sulfur (S) and/or oxygen (O₂) or others, from entering or re-entering into the melted metal material by pushing them away from the melted material as the protective gas is coming out of the melt. The additive(s) that forms the protective gas atmosphere can also react with the dissolved sulfur in the melt and/or the oxides that were in suspension in the melt before the introduction of the additive(s). Reaction of the additive(s) with the dissolved sulfur in the melt will increase the sphericity of the particles and/or the microstructural phases and constituents. The reaction of the additive(s) with oxides that were in suspension in the melt before the introduction of the additives will lower the amount and size of internal porosities. The reaction of the additive(s) with both oxides and dissolved sulfur that were in the melt before the introduction of the additives will lower the amount and size of internal pores and increase the sphericity of the particles and/or the microstructural phases and constituents. In some cases, reaction between the additive(s) and the dissolved sulfur will also lower the amount and size of internal pores. The particles formed by the improved method are either cleaner, and/or contain less internal pores, and/or are more spherical, and/or include more spherical microstructural constituents and/or phases.

When water atomization is employed, adding the additive(s) to the melted metal material can increase the sphericity of the atomized particles to a level approaching the sphericity of particles formed by gas atomization, but with reduced costs compared to gas atomization. Adding the additive(s) to the melted metal material can also produce cleaner particles by limiting the formation and the entrainment of new oxides from the surface of the melt and by reacting with those already present in the melt before the introduction of the additive(s). These oxides can form as bifilms where films of oxides are superimposed leaving a weak interface in between the oxide films. The additive(s) can also lower the amount and size of internal porosity, a problem encountered in atomized powders. The additive(s) can also increase the sphericity of microstructural constituents and/or phases formed in the atomized particles and/or during a subsequent heat treatment process. For example, if the atomized particles are formed from a cast iron material, at least 50% of the graphite precipitates formed during the post heat treatment process will have a circularity of at least 0.6 and a roundness of at least 0.6.

According to one example embodiment, the method begins by melting a base metal material. Many different metal compositions can be used as the base metal material. However, in order to produce enough gas that will act as a protective atmosphere and thus obtain either the desired spherical-shape of the powders and/or more spherical microstructural constituents and/or cleaner particles and /or having less internal pores, the additive(s) must have a low solubility in the metal material. The base material and the additive(s) should be selected such that when the additive(s) are introduced, the volume of protective gas atmosphere generated is at least three times the volume of melt to be treated. For example, if 0.22 weight percent (wt. magnesium is added to a melt having a composition similar to that of FGP4025Mg of FIG. 7, the generated volume of gas is calculated to be about 20 times the inferior volume limit.

The base metal material typically includes at least one of aluminum (Al), copper (Cu), manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe), titanium (Ti), and chromium (Cr). The base material can comprise pure Al, Cu, Mn, Ni, Co, Fe, Ti, or Cr. Aluminum-rich, copper-rich, manganese-rich, nickel-rich, cobalt-rich, iron-rich, titanium-rich and chromium-rich alloys, or an alloy including at least 50 wt. % of Al, Cu, Mn, Ni, Co, Fe, Ti, and/or Cr are also well suited for use as the starting base metal material. Mixtures of these base metal materials in different proportions are also well suited for use as the starting material such as, but not limited to, Al—Cu, Fe—Ni, Fe—Co, Fe—Ni—Co, Ni—Cr, Ti—Cu, and Co—Cr alloys. The alloys can also include at least one of the following as alloying elements, as long as they will stay in solution in the melt of the alloy of interest: silver (Ag), boron (B), barium (Ba), beryllium (Be), carbon (C), calcium (Ca), cerium (Ce), gallium (Ga), germanium (Ge) potassium (K), lanthanum (La), lithium (Li), magnesium (Mg), molybdenum (Mo), nitrogen (N), sodium (Na), niobium (Nb), phosphorus (P), sulfur (S), scandium (Sc), silicon (Si), tin (Sn), strontium (Sr), tantalum (Ta), vanadium (V), tungsten (W), yttrium (Y), zinc (Zn), and zirconium (Zr). According to one example embodiment, to create a gaseous protective atmosphere in an Al—Mg alloy, K and/or Na should be used as an additive and the melt temperature should be selected according to the selected additive(s), see FIG. 28. Mg is used as an alloying element in aluminum alloys (the Al-5000 series) and will not generate a protective gas atmosphere.

However, the starting metal material is not limited to the above mentioned compositions. Other metal compositions can be used, as long as the additive has a low solubility in the selected material and generates a sufficient amount of protective gas atmosphere. Also, in order for the additive treatment to be effective in changing the shape of the powders and microstructural constituents and/or phases, the additive must react with impurities, such as sulfur present in the melted metal material to reduce the amount of sulfur in solution in the melt and thus increase surface tension. The surface area of high surface tension liquid or solid constituents exposed to the surrounding environment is minimized when the constituents adopt a spherical shape, which in turn minimizes the surface to volume ratio. Some additives that are used to create the gaseous protective atmosphere will naturally react with the dissolved sulfur in the melt to create more stable compounds and thus increase the surface tension. This is the case for Mg in Fe-rich systems in which solid MgS will precipitate. However, some additives will create a protective atmosphere but will not react with the dissolved sulfur, as is the case with Na in Fe-rich systems. In these situations, a combination of different additives must be used to increase surface tension.

Sulfur can be used as an alloying element in different chemical systems to create solid sulfides in the atomized powders (known as powders with prealloyed sulfides). These sulfides are often desired to improve machinability of the parts made with these powders. To create more spherical particles that contain prealloyed sulfides, the amount of additive(s) that will create a protective gas atmosphere and that react with the dissolved sulfur to increase the sphericity of the particles and/or microstructural phases and constituents should be increased according the amount of sulfides that are desired. For instance, from calculations it was determined that adding 0.70 wt. % Mg to an Fe-rich alloy that contains 1.4%C, 1.1%Si and 0.50 wt. % S will create about 18 times to inferior limit of gas, will create about 0.90 wt. % sulfides (MgS) and will lower the amount of dissolved sulfur in the melt to increase the sphericity of the particles

As mentioned above, various different additives could be added to the melted metal material to achieve the increased protective atmosphere and the other advantages mentioned above. The additive(s) selected depends on the composition of the base metal material. For example the at least one additive can include at least one of K, Na, Zn, Mg, Li, Ca, Sr, and Ba. The protective gas atmosphere generated by the additive(s) prevents impurities from entering or re-entering into the melted metal material.

The additives listed above generate different amounts of protective gas atmosphere, depending on the chemical system in which they are used. Some additives are more suited for some systems than others. For example, in aluminum alloys, K and Na are oftentimes preferred. In copper alloys, K and Na are oftentimes preferred. In manganese alloys, K, Na, Zn, Mg, and Li are oftentimes preferred. In nickel alloys, K and Na are oftentimes preferred. In cobalt alloys, K, Na, Li, and Ca are oftentimes preferred. In iron alloys, K, Na, Zn, Mg, Li, Sr, and Ca are oftentimes preferred. In titanium alloys, Zn, Mg, Li, Ca, and Ba are oftentimes preferred. In chromium alloys, K, Na, Zn, Mg, Li, Sr, Ca, and Ba are oftentimes preferred. Examples are provided in FIG. 37.

In addition to the generation of a protective atmosphere, if more spherical particles and microstructural phases and/or constituents are desired, the additive(s) must react with the dissolved sulfur. Some additives are more effective in some systems than others. According to one embodiment, the same additive can form the protective atmosphere and react with the sulfur. According to another embodiment, an additional additive is added to react with the sulfur present as an impurity and already dissolved in the melted base metal material. This additional additive could contribute to the protective gas atmosphere, but will not necessarily create the protective gas atmosphere, in which case it must be coupled with another additive that can create the protective gas atmosphere.

When the melted base material is iron-based and includes sulfur as an impurity, Zn, Mg, Li, Sr, Ca, and Ba are preferred to react with the sulfur. An example of such a combination of additives in an iron-based material or Fe-rich alloy to create more spherical particle and/or phases and constituents could be a mixture of Na and Ba. Na will create a protective atmosphere and Ba will react with S. When the melted base metal material is a titanium alloy or titanium-based and includes sulfur as an impurity, K, Na, Zn, Mg, Li, Sr, Ca, and Ba are preferred to react with the sulfur. When the melted base metal material is a cobalt alloy or cobalt-based and includes sulfur as an impurity, Na, Mg, Li, Sr, Ca, and Ba are preferred to react with the sulfur. When the melted base metal material is a chromium alloy or chromium based and includes sulfur as an impurity, K, Na, Zn, Mg, Sr, Ca, and Ba are preferred to react with the sulfur. When the melted base metal material is an aluminum alloy or aluminum-based and includes sulfur as an impurity, K, Na, Mg, Li, Sr, Ca, and Ba are preferred to react with the sulfur. When the melted base metal material is a nickel alloy or nickel-based and includes sulfur as an impurity, Mg, Li, Sr, Ca, and Ba are preferred to react with the sulfur. When the melted base metal material is a copper alloy or copper-based and includes sulfur as an impurity, K, Na, Mg, Li, Sr, Ca, and Ba are preferred to react with the sulfur. When the melted base metal material is a manganese alloy or manganese-based and includes sulfur as an impurity, K, Na, Mg, Li, Sr, Ca, and Ba are preferred to react with the sulfur. Examples are provided in FIG. 38.

According to one specific example embodiment, the metal base material is iron-rich and includes Mg which generates the protective gas and also reacts with the sulfur impurity. Alternatively, the base metal material is pure iron and the additive is Mg. According to another specific example, the metal base material is iron-rich and the additives include a mixture of K and Ba. The potassium (K) is expected to generate the protective gas atmosphere, and the barium (Ba) is expected to react with the sulfur.

The protective atmosphere limits the amount of oxides in the atomized particles and will also limit the size and amount of internal porosities. Some additives that are used to create the gaseous protective atmosphere will naturally react with oxides that are in suspension in the melt to create more stable compounds and will also change their morphology during the chemical reaction process, for example a Mg additive in Fe-rich systems that contain Si as an alloying element. In these materials, oxides of SiO₂ that could be in the form of bifilms are in suspension in the melt. One of the reason explaining that smaller amount of porosities are observed is that Mg is sealing the interfaces of the bifilms as a result of the chemical reaction between Mg and the oxides, creating a stronger interface that cannot be further inflated to form pores. See examples of lower amount of porosities in particles of stainless steel smaller than 177 microns by a Mg treatment in FIGS. 16 and 17. The self-generated Mg gaseous atmosphere will limit further oxidation of the surface of the melt, which will limit the amount of internal oxides in the particles. However, some additives will create a protective atmosphere but will not react with the oxides in suspension in the melt, as is the case of Zn in Ti-rich systems. In these situations, a combination of different additives must be used to limit the amount and size of internal porosities. For example, at least one additive could be added to generate the protective gas atmosphere that will prevent impurities from entering or re-entering into the melted metal material, and at least one additive could be added to react with the oxides already in the melt but would not necessarily create a protective gas atmosphere. An example of such a combination of additives in a Ti-rich alloy to create more spherical particle and/or phases and constituents having less internal porosities could be a mixture of Zn to create a protective atmosphere and Sr to react with S and with TiO₂ but without participating in the generation of the protective atmosphere.

In other words, some additives are more effective in some systems than in others, depending on the type of oxides that are formed. As indicated above, if less internal porosities with smaller sizes are desired, the additive(s) must react with the oxides in suspension in the melt. These oxides are also considered impurities in the melted base metal material, for example, Al₂O₃ in an aluminum-based material, or Fe₂O₃ in an iron-based material. When the melted base metal material is an aluminum alloy or aluminum-based, the preferred additives to react with the oxides include K, Na, Mg, Li, and Ca. When the melted base metal material is an iron alloy or iron-based, the preferred additives to react with the oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba. When the melted base metal material is a titanium alloy or titanium-based, the preferred additives to react with the oxides include Sr, Ca, and Ba. When the melted base metal material is a chromium alloy or chromium-based, the preferred additives to react with the oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba. When the melted base metal material is a cobalt alloy or cobalt-based, the preferred additives to react with the oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba. When the melted base metal material is a copper alloy or copper-based, the preferred additives to react with the oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba. When the melted base metal material is a manganese alloy or manganese-based, the preferred additives to react with the oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba. When the melted base metal material is a nickel alloy or nickel-based, the preferred additives to react with the oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba. Examples are provided in FIG. 39.

In addition, certain additives will successfully generate the protective gas atmosphere, and also react with the sulfur and oxides present as impurities in the melted base metal material. For example, when the melted base metal material is an iron-alloy or iron-based, additives that will generate the protective gas atmosphere and react with the sulfur and oxide impurities include Zn, Mg, Li, Sr, and Ca. When the melted base metal material is a titanium alloy or titanium-based, additives that will generate the protective gas atmosphere and react with the sulfur and oxide impurities include Ca and Ba. When the melted base metal material is a chromium alloy or chromium-based, additives that will generate the protective gas atmosphere and react with the sulfur and oxide impurities include K, Na, Zn, Mg, Sr, Ca, and Ba. When the melted base metal material is a cobalt alloy or cobalt-based, additives that will generate the protective gas atmosphere and react with the sulfur and oxide impurities include Na, Li, and Ca. When the melted base metal material is an aluminum alloy or aluminum-based, additives that will generate the protective gas atmosphere and react with the sulfur and oxide impurities include K and Na. When the melted base metal material is a copper alloy or copper-based, additives that will generate the protective gas atmosphere and react with the sulfur and oxide impurities include K and Na. When the melted base metal material is a manganese alloy or manganese-based, additives that will generate the protective gas atmosphere and react with the sulfur and oxide impurities include K, Na, Mg, and Li.

As stated above, the powder metal material can be manufactured by water or gas atomization. Furthermore, some metal materials are less suited for water atomization and other atomization processes such as gas and plasma atomization are preferred. For example, titanium reacts readily with oxygen and can form titanium oxide, a very stable compound, and thus water atomization of titanium alloys is not preferred. Titanium alloy powders are more commonly produced by gas atomization and plasma atomization. In this case, the at least one additive that would be used is, for instance, calcium (Ca) which would also react with the dissolved S. This could provide conditions that will favor more aggressive atomization parameters to lower the size distribution of the powder and improve the yield of small spherical powders. Ca would also react with any residual oxygen that would be present in the process and thus lower the amount and size of internal porosities.

As alluded to above, the starting base metal material selected oftentimes includes iron in an amount of at least 50.0 wt. %, based on the total weight of the metal material before adding the additive(s). For example, cast iron, highly alloyed cast iron, stainless steel, unalloyed and alloyed steel, tool steel, Maraging steel, or Hadfield steel could be used. According to one example embodiment, the metal material is a steel powder including 1.3 wt. % carbon and 1.1 wt. % silicon. According to another example embodiment, the metal material is a cast iron powder including 4.0 wt. % carbon and 2.3 wt. % silicon. According to another example embodiment, the metal material is a stainless steel powder including 1.2% Mn, 0.30% Si, 0.44% Cu, 0.23% Mo, 17.3% Cr, 9.5% Ni, and other trace elements. As stated above, aluminum alloys (for instance the alloys designated as 2024, 3003, 3004, 6061, 7075, 7475, 5080 and 5082), copper alloys (such as aluminum bronzes, silicon bronzes, and brass), manganese alloys, nickel alloys (for instance the alloy designated as 625), cobalt alloys (such as tribaloy and Haynes188), cobalt-chromium alloys (such as CoCrMo alloys and stellite), titanium alloys (for instance the alloys designated as Ti-6Al-4V), chromium alloys (such as the Kh65NVFT alloy) and any hybrid alloys made from these chemical systems can also be used as the starting powder metal material (for instance, alloys designated as Invar, Monel, Chromel, Alnico, and Nitino160). These examples are not exhaustive and other metal compositions can be used, as long as the at least one additive (potassium (K), sodium (Na), zinc (Zn), magnesium (Mg), lithium (Li), strontium (Sr), calcium (Ca), and barium (Ba)) has low solubility in the selected material, such that a protective gas atmosphere is formed on top of the melted material to form a total amount of at least three times the initial volume of melt to be treated. FIGS. 26-36 represent the results of calculations and experiments conducted which show the increased volume of protective gas atmosphere generated when the additive(s) are added to the melted metal material according to example embodiments of the invention. FIG. 26 presents a curve of the total volume of gas that is obtained as a function of the amount of additive(s) for an example composition. The additive (here, the additive was a mixture of 90 wt. % Mg and 10 wt. % Na). The alloy is a cast iron material (Fe-rich) that contains 4.0% C, 1.5% Si, 0.02% S and 2.0% Cu. This curve was calculated using the chemical composition of one powder that was water atomized, the amount of additive used in this experiment was 0.11 wt. %, which resulted in about 0.40 liter of protective gas (Mg and Na) for each 100 grams of melt. The dashed line represents the inferior limit of the total amount of gas that should be obtained to provide a protective atmosphere which has a volume that is three times the initial volume of melt to be treated. In this specific example, the calculated amount of gas is about five times the inferior limit.

FIG. 27 presents Energy-dispersive X-ray spectroscopy (EDS) spectra that were acquired on a polished pure iron surface before and after it was exposed to the gaseous atmosphere on top of the tundish during the atomization process of the powder that is described in FIG. 26. This confirms that the additives (in this case Mg and Na) formed a gaseous protective atmosphere that was generated on top of the melt and that these elements deposited on the exposed polished iron surface;

FIG. 28 presents examples of different amounts of gas that can be generated in aluminum alloys for different additives at different temperatures. The base system for calculations is Al+0.02% S+0.02% Al2O3. The dashed line represents the inferior limit of the amount of gas that should be obtained to provide a protective atmosphere which is defined as three times the initial volume of melt to be treated. In these examples, the minimum amount of additive to be added varies according to the nature of the additive and the temperature of the melt. For instance, Na cannot generate enough gas if the melt is at a temperature of about 800 Celsius, regardless of the amount that is added. However, if the temperature of the melt is increased to about 900 Celsius, the minimum amount of Na is about 0.32 wt. % to generate at least three times the initial volume of melt to be treated. For K, the minimum amount is 0.36 wt. % if the melt is at 800 Celsius, and 0.26 wt. % if the melt is at about 900 Celsius. If a mixture of half Na and half K is used in an aluminum melt at 900 Celsius, the minimum amount of Na+K will be about 0.29 wt. % (0.16 wt. % Na and 0.13 wt. % K). FIG. 29 presents examples of the minimum amount of different additives to be added to a titanium melt at 1800 Celsius. For instance, an addition of 0.11 wt. % Ca will provide about the same minimum amount of gas protection as an addition of 0.48 wt. % Zn. Similarly, FIGS. 30 to 35 present other examples of the minimum amount of different additives in different systems (Co, Cr, Cu, Fe, Mn, and Ni). FIG. 36 presents the calculated minimum amount of additive (K+Li) in a complex cobalt alloy.

After adding the at least one additive to the melted base metal material, the method next includes atomizing the melted metal material. Gas atomization, water atomization, plasma atomization, or rotating disk atomization can be used. However, water atomization is oftentimes preferred because it is three to nine times less expensive than gas atomization and even less expensive than the other atomization processes. However, for some alloys that are readily oxidized, gas atomization is preferred. An additive treatment before gas atomization could allow improved conditions for atomization such as larger gas pressures and still achieve round particles and could also limit the amount of internal oxides and porosities. In addition, the added additive(s) can increase the sphericity of the water atomized particles, such that the sphericity approaches the sphericity of gas atomized particles.

The water atomizing step typically includes applying water to the melted metal material at a given pressure. In an example embodiment, the pressure ranges from 2.6 to 7.5 MPa since the atomization was performed with a laboratory scale atomizer and a limited available pressure range. However, other pressure levels can be used depending on the composition and process parameters used. For example, the pressure of the atomizing step could be from about 2 MPa to about 150 MPa and above. An external protective atmosphere or vacuum system can also be used together with the self-generated protective atmosphere describe herein such as, but not limited to: projection of a flow of nitrogen (N₂), or the projection of an argon stream on top of melt. The melt could also be enclosed in a chamber with a vacuum system. These systems can increase the effectiveness of the process.

During or before the atomization step, the method includes adding the additive(s) to the melted metal material. In other words, the method includes atomizing the melted metal material after adding at least some of the additive(s). As discussed above, the additive(s) is added in an amount such that the total volume of gas after the introduction of the additive(s) is at least three times the initial volume of the melt to be treated. In one example embodiment, specifically alloy FGP1210Mg of FIG. 7, the additive, in this case, Mg, is added in a single operation as lumps of pure Mg in an amount ranging from 0.05 to 1.0 wt. %, for example 0.18 wt. %, based on the total weight of the melted base metal material and the added magnesium. Thus, the resulting atomized powder metal material includes a very low amount of residual magnesium and a total sulfur content similar to the powder without the additive but for which S is now chemically bounded with the additive (as solid precipitates of MgS) and not dissolved in the melt, which leads to a larger surface tension and thus more spherical particles. Thermodynamical calculations showed that the free sulfur content in the Mg-treated powder was more than 10 times lower than that of the non-treated powder, even if the total sulfur content for both powders were similar.

The additive(s) can be added in a single continuous step, for example up to 1.0 wt. % in a single continuous step, or multiple steps spaced from one another by a period of time, for example three or four steps each including up to 0.2 wt. % of the additive(s). If only one continuous magnesium treatment is applied to the melted base metal material, then the atomization step typically lasts from 10 to 30 minutes. However, the atomization step could be conducted for a longer period of time if atomization of larger melts is conducted. When water atomizing an iron-based material, the additive(s) is typically added before the water atomization process, such that the atomization step occurs after the violent reaction of the additive(s) with liquid iron. The additive(s) can be added in the furnace or in a ladle and they can be in the form of pure metal, or as an alloy or compound including the additive(s). Different techniques that are already available can be used to introduce the additive(s) to the melted metal materials such as, but not limited to, lumps/chunk of the material that contains the additive(s) can be directly deposited on top of the melt, or the usage of the cored wire technique or the usage of the plunger process.

The atomizing step can also include producing a plurality of particles having a spherical shape. The sphericity of the particles can be calculated by two image analysis indicators, specifically circularity and roundness, according to the following formulas:

Circularity (C)=4π×([Area]/[Perimeter]²)

Roundness (R)=4×([Area]/(π×[Major axis]²))=1/AR

wherein AR=[Major axis]/[Minor axis].

The image analysis indicators can be calculated using open source software, ImageJ (http://imagej.nih.gov/ij/). A sphericity index value of 1.0 indicates a perfect circle.

When the additive(s) react with the dissolved sulfur, the median of the circularity of the powder metal material formed by the method described above is at least 0.60, and the median of the roundness of the powder metal material formed by the method described above is also at least 0.60. More preferably, the median of the circularity and that of the roundness of the powder metal material formed by the method described is at least 0.64, and even more preferably the median of the circularity and that of the roundness of the powder metal material is at least 0.68.

As stated above, by adding the additive(s) to the melted metal material (in the case of Mg in Fe-rich alloys), the number of water atomized particles that have a circularity and a roundness value of 0.6 and larger increased by at least 8%, compared to the same water atomized material without the additive(s). The additive(s), for example magnesium, also results in fewer internal oxides, and could seal the interface of residual oxide bifilms present in the melted metal material, which in turn produces cleaner atomized particles having less and smaller internal porosities.

FIGS. 1-6 are photomicrographs illustrating the improved sphericity achieved by adding magnesium before or during the water atomization process. Each of the Figures shows Si-alloyed steels, cast iron powders and stainless steels (type 304) that were screened at −200 mesh (74 microns and smaller). The materials shown in FIGS. 1 and 2 were water atomized and include 1.3 wt. % carbon and 1.1 wt. % silicon. The material of FIG. 1 was atomized without the added magnesium, according to an example embodiment of the invention, while the material of FIG. 2 was atomized with the added magnesium. The median of the circularity of the inventive powder metal particles shown in FIG. 2, with the added magnesium, was calculated to be 0.81. The median of the circularity of the comparative metal particles shown in FIG. 1, without the magnesium, was calculated to be 0.71. The median of the roundness of the inventive powder metal particles shown in FIG. 2, with the added magnesium, was calculated to be 0.72. The median of the roundness of the comparative metal particles shown in FIG. 1, without the magnesium, was calculated to be 0.63. The improvement of the median of the circularity and that of the roundness were also observed for the other experimentally tested powders, as shown in FIGS. 14 and 15. In summary, adding at least one additive, for example magnesium that forms a protective atmosphere and reacts with the dissolved sulfur will increase the sphericity (circularity and roundness) of the water atomized particles to a level approaching the sphericity of gas atomized powders.

After the atomization step, the method can include a post heat treatment process. The heat treating step can include annealing or another heating process typically applied to powder metal materials. The heat treatment is conducted in an inert or reducing atmosphere, such as but not limited to an atmosphere including nitrogen, argon, and/or hydrogen or vacuum. For example, annealing in a reducing atmosphere after water atomization can reduce surface oxides. The heat treatment step can also include forming microstructural phases and/or constituents in the atomized particles, for example graphite precipitates or nodules, carbides, or nitrides. Other microstructural phases and/or constituents could be present, depending on the composition of the metal material. In one example embodiment, the metal material is a hypereutectic cast iron alloy, and the cementite present in the cast iron alloy transforms into ferrite and spheroidal graphite nodules during the heat treatment step. Spherical carbides should also be formed during the heat treatment of highly alloyed steel.

The additive(s) can also increase the sphericity of the microstructural constituents and/or phases formed in the atomized particles during post heat treatment. However, rounder phases and/or constituents could be present in the powder metal material directly after atomization and not only after heat treatments. The microstructural phases can include graphite precipitates, carbides, and/or nitrides. Other microstructural phases and/or constituents could be present, depending on the composition of the metal material. Typically, the microstructural constituents and/or phases have a median of the circularity and a median of the roundness of at least 0.6. Also, there is at least 10% more, and preferably at least 15% more constituents and/or phases formed in the magnesium-treated material that have a circularity and a roundness value larger than 0.6 compared to those of the same alloy but without the additive treatment.

According to one example embodiment, the powder metal material includes iron, such as cast iron, in an amount of at least 50 wt. %, and the atomized particles include graphite precipitates, wherein at least 50% of the graphite precipitates have a circularity and a roundness value of 0.6 and greater. In another embodiment, wherein the metal material is iron-based, for example alloys FGP4025 and FGP4025Mg of FIG. 7, the annealing step includes producing graphite precipitates or nodules, and the graphite precipitates or nodules have a median of the circularity and a median of the roundness of at least 0.6. In one example embodiment, the metal material is a hypereutectic cast iron alloy, and spheroidal graphite nodules are formed during the heat treatment process.

FIGS. 20 and 21 are photomicrographs illustrating the improved sphericity of the microstructural phases and/or constituents, specifically graphite nodules, achieved by adding an additive (in this case magnesium) before or during the water atomization process and after heat treatment. Each material is a cast iron powder including about 4.0 wt. % carbon and 2.3 wt. % silicon. However, the material of FIG. 20 was atomized without the added magnesium, while the material of FIG. 21 was atomized with the added magnesium. The median of the roundness of the graphite nodule shown in FIG. 20, without the added magnesium, was calculated to be 0.56. The median of the roundness of the graphite nodule with magnesium shown in FIG. 21, was calculated to be 0.73. Other results that show the improved sphericity of the nodules by the additive treatment are presented in FIGS. 24 and 25.

If the atomized particles do not comprise the desired particle size or morphology after the atomization process, then the method can include milling the atomized particles. For example, the atomized particles can be milled to change the morphology from spherical to irregular and to improve green strength.

The added magnesium is also expected to lower the internal porosity of the particles. For instance, FIG. 16 presents a large amount of larger internal porosities in a stainless steel powder (SS304, screened at −80 and +200 mesh) without any additive (see the chemistry in FIG. 7). However, by adding 0.15 wt. % Mg to the melt, the amount and size of internal porosities in the atomized powders were significantly reduced, as shown in FIG. 17. Observation of about 260 particles for each powder (SS304 and SS304Mg) shows that the number of particles that contain internal porosities went from 17% to 8%, thus an improvement of more than 50%. The number of internal oxides was also measured and went from 15% to about 10%, thus an improvement of about 33%.

As stated above, the self-generated protective atmosphere created after the introduction of the additive(s) will inhibit the oxidation of the surface of the melt and will limit the amount of internal oxides in the powders. FIG. 18 shows primary graphite nodules that precipitated on silicon oxides that were formed during pouring from the crucible to the tundish and were in suspension in the melt prior to the atomization of the FGP4025 powder. In Fe-rich systems that contain a high carbon content, carbon provides a protection against oxidation of the melt in the crucible (because of the high temperature), which prevents the formation of oxides in the crucible. Numerous graphite nodules that grew on these different oxides can be observed in the powder without an additive. By comparison, FIG. 19 presents one of the few primary graphite nodules that can be observed in the alloy that was treated with an additive (Mg in the FGP4025Mg alloy of FIG. 7). Since the protective atmosphere made of Mg gas limited the oxidation of the melt directly from the crucible and throughout pouring, the amount of oxides that were present in the melt before the introduction of the additive was significantly less than in the melt without the additive. Thus, very few substrates were available for graphite precipitation during the atomization of the FGP4025Mg powder.

As described above, due to the combined effect of the additive(s) on the generation of a protective atmosphere and the reaction with dissolved sulfur, the atomized particles have a spherical shape, even when produced by water atomization. The median of the circularity and that of the roundness of the atomized particles is at least 0.6. The particle size of the atomized particles can vary. According to one embodiment, the atomized particles have a particle size or diameter of not greater than 2.5 mm. For example, when the FPG4025(Mg) compositions were atomized with a water pressure of 2.6 MPa, particles with a maximum diameter of the order of about 2 mm were obtained. According to another embodiment, the atomized particles have a particle size of not greater than 500 microns. For example, the atomized particles can be screened at −200 mesh (74 microns and less). According to another embodiment, when the SS304(Mg) compositions were atomized with a water pressure of 7.5 MPa, particles with a maximum diameter of the order of about 400 microns were obtained with a median size of about 72 microns. It is also possible to further vary the water pressure and/or to screen the atomized powders to a different size and obtain a size distribution that fits the targeted process including additive manufacturing.

The powder metal material is typically formed by water or gas atomization. However another atomization process can be used in various different automotive or non-automotive applications. For example, the atomized particles can be used in typical press and sinter processes. The atomized particles can also be used for metal injection molding, thermal spraying, and additive manufacturing applications such as three-dimensional printing and selective laser sintering.

Experiment

The sphericity, observation of internal porosities and internal oxides of powder metal materials having the compositions shown in the Table of FIG. 7 were measured after a water atomization process. Four of the compositions included magnesium added to the melted metal material before the atomization step and three of those were compared to the same material without the added magnesium. For each of these powders, about 15 to 25 kilograms of the raw materials were melted in an induction furnace. A flow of argon was projected on top of the melt throughout the atomization process. Then, the Mg was added as pure Mg for the silicon steel designated as FGP1210Mg and the cast iron designated as FGP4025Mg, as FeSiMg(3.65 wt. % Mg) plus an addition of about 0.01 wt. % Na for the cast iron designated as S4-FGP#1 and as NiMg(15 wt. % Mg) for the stainless steel powder (SS304Mg). The atomization temperature was about 1550 Celsius for the silicon steel, about 1500 Celsius for the cast iron FGP4025Mg, about 1620 Celsius for the cast iron S4-FGP#1 and 1640 Celsius for the stainless steel. The water pressure was 4.5 MPa for the silicon steel, 2.6 MPa for the cast iron FGP4025Mg, 5.0 MPa for the cast iron S4-FGP#1 and 7.5 MPa for the stainless steel. For the four powders treated with Mg, the atomization was completed in about 10 to 20 minutes after the Mg addition. While the above details were performed in laboratory, similar mechanisms and trends will translate to an industrial environment.

FIG. 8 illustrates the circularity frequency distribution of the FGP1210 and the FGP1210Mg powders screened at −200 mesh. FIG. 9 illustrates the roundness frequency distribution of the FGP1210 and the FGP1210Mg powders screened at −200 mesh. FIG. 10 illustrates the circularity frequency distribution of the FGP4025 and the FGP4025Mg powders screened at −200 mesh. FIG. 11 illustrates the roundness frequency distribution of the FGP4025 and the FGP4025Mg powders screened at −200 mesh. FIG. 12 illustrates the circularity frequency distribution of the SS304 and the SS304Mg powders screened at −200 mesh. FIG. 13 illustrates the roundness frequency distribution of the SS304 and the SS304Mg powders screened at −200 mesh. FIG. 14 is a table illustrating numerical data for the circularity of each composition listed in the table of FIG. 7. FIG. 15 is a table illustrating numerical data for the roundness of each composition listed in the Table of FIG. 7. Since Mg reacted with the dissolved sulfur in all these systems, an improvement of the circularity and roundness is observed for all the powders that were treated with this additive.

FIG. 20 presents graphite nodules which grew in a solid state during a post heat treatment process of the cast iron powder FGP4025 without added Mg. By comparison, FIG. 21 presents more spherical graphite nodules which grew in a solid state during a post heat treatment process of the cast iron powder FGP4025Mg (with added Mg). The two powders were treated in the same furnace with the same heat treatment profile. FIG. 22 illustrates the circularity frequency distribution of the graphite nodules that grew in the solid state in the cast iron powders FGP4025 and FGP4025Mg. FIG. 23 illustrates the roundness frequency distribution of the graphite nodules that grew in the solid state in the cast iron powders FGP4025 and FGP4025Mg. FIG. 24 is a table illustrating numerical data for the circularity of the graphite nodules that grew in the solid state in the cast iron powders FGP4025 and FGP4025Mg. FIG. 25 is a table illustrating numerical data for the roundness of the graphite nodules that grew in the solid state in the cast iron powders FGP4025 and FGP4025Mg. Since Mg reacted with the dissolved sulfur in the FGP4025Mg powder, an improvement of the circularity and roundness of the graphite nodules that grew in the solid state during a post heat treatment process was observed compared to the graphite nodules present in the powder that was not treated with Mg (FGP4025).

FIG. 16 shows numerous internal porosities in the SS304 without an additive. FIG. 17 shows that the amount of internal porosities was lowered compared to those of FIG. 16 by the introduction of Mg in the melt before the atomization. Observation of about 260 particles for each powder (SS304 and SS304Mg) shows that the number of particles that contain internal porosities went from 17% to 8%, thus an improvement of more than 50%. The number of internal oxides was also measured and went from 15% to about 10%, thus an improvement of about 33%. Note that the exact values of the improvement of the amount of internal oxides and internal porosities are dependent on the alloy, the atomization process and the process parameters. FIG. 18 shows many irregular primary graphite nodules in the cast iron powder FGP4025 (without added magnesium) that precipitated on internal silicon oxides that were introduced in the melt during the pouring step of the atomization process. By comparison, FIG. 19 presents one of the few primary graphite nodules that can be observed in the cast iron powder FGP4025Mg (with added Mg). The protective gas atmosphere of Mg limited the oxidation of the melt directly from the crucible and throughout pouring, and the amount of oxides that were present in the melt before the introduction of the additive was significantly less than in the melt without the additive. This is demonstrated by the very limited amount of substrates available for graphite precipitation during the atomization of the FGP4025Mg powder.

FIG. 27 presents EDS spectra that were experimentally acquired on a polished pure iron surface before and after it was exposed to the atmosphere on top of the tundish after pouring the melt of the powder S4-FGP#1. In this case, the additives were Mg and Na. The spectra of FIG. 27 prove that a protective gaseous atmosphere made of Mg and Na was formed on top of the melt. FIG. 26 presents the calculated volume of protective gas that was formed for each 100 grams of a melt that has a composition similar to the S4-FGP#1 alloy. The amount of protective gas that was formed for an addition of 0.11 wt. % Mg+Na is about 5 times the inferior volume limit.

FIGS. 28 to 35 show the calculated volume of protective gas that is formed for each 100 grams of melt of different pure metals (Al, Ti, Co, Cr, Cu, Fe, Mn, and Ni) and for different amounts of various additives. These figures indicate that for one particular chemical system, the minimum amount of an additive that must be added to create a protective gas atmosphere varies according to the nature of the additive. For instance, in iron at 1650 Celsius, the minimum amount of Zn to create a protective gas atmosphere made of Zn is about 0.20 wt. %, but the minimum amount of Li to create a protective gas atmosphere made of Li is about 0.06 wt. %. FIGS. 28 to 35 also show that the minimum amount of one particular additive to create a protective gas atmosphere varies according to the chemical system in which it is used. For instance, in iron at 1650 Celsius the minimum amount of Zn to create a protective gas atmosphere made of Zn is about 0.20 wt. %, but in titanium at 1800 Celsius the minimum amount of Zn to create a protective gas atmosphere made of Zn is about 0.50 wt. %.

FIG. 36 shows the calculated volume of protective gas that is formed for each 100 grams of melt of a complex cobalt alloy than contains various alloying elements (28% Cr, 6 % Mo, 0.5% Si, 0.5% Fe, 0.5% Mn, and 0.02% S) and 0.02 wt. % chromium oxides in the melt (Cr₂O₃) at 1600 Celsius. The additive that forms the protective gas atmosphere is a mixture made of 60 wt. % K and 40 wt. % Li. For this system, the minimum amount of additive to create a volume of gas that is at least 3 times the volume of melt to be treated is about 0.025 wt. % K+Li (0.015 wt. % K and 0.010 wt. % Li). For instance, if 0.10 wt. % K+Li is added, the calculations showed that the volume of the protective gas atmosphere is about 5 times the inferior limit and that the composition of the protective gas is about 66 vol. % K and 27 vol. % Li. Calculations also showed that the additive Li reacted with the dissolved sulfur and the chromium oxides. The examples presented in FIGS. 26 to 36 are not exhaustive and not inclusive.

Based on the experiment, when the additive(s) react with the dissolved sulfur, it was concluded that the minimum percentage of particles in bin ]0.7-1.0] for the circularity and the roundness is typically 30%. More preferably, the minimum percentage of particles in bin ]0.7-1.0] for the circularity and the roundness is 40%. Even more preferably, the minimum percentage of particles in bin ]0.7-1.0] for the circularity and the roundness is 50%.

Based on the experiment, when the additive(s) react with the dissolved sulfur, it was concluded that the minimum percentage of particles in bin ]0.8-1.0] for the circularity and the roundness is typically 15%. More preferably, the minimum percentage of particles in bin ]0.8-1.0] for the circularity and the roundness is 20%. Even more preferably, the minimum percentage of particles in bin ]0.8-1.0] for the circularity and the roundness is 25%.

Based on the experiment, when the additive(s) react with the dissolved sulfur, it was concluded that for the circularity and roundness, the minimum relative percentage increase of particles in bin ]0.6-1.0] is typically 8% compared to the powder without the additive. More preferably, for the circularity and roundness the minimum relative percentage increase of particles in bin ]0.6-1.0] is 10%. Even more preferably, for the circularity and roundness the minimum relative percentage increase of particles in bin ]0.6-1.0] is 12%.

Based on the experiment, when the additive(s) react with the dissolved sulfur, it was concluded that for the circularity and roundness, the minimum relative percentage increase of particles in bin ]0.7-1.0] is typically 15% compared to the powder without the additive. More preferably, for the circularity and roundness the minimum relative percentage increase of particles in bin ]0.7-1.0] is 20%. Even more preferably, for the circularity and roundness the minimum relative percentage increase of particles in bin ]0.7-1.0] is 25%.

Based on the experiment, when the additive(s) react with the dissolved sulfur, it was concluded that for the circularity and roundness, the minimum relative percentage increase of particles in bin ]0.8-1.0] is typically 20% compared to the powder without the additive. More preferably, for the circularity and roundness the minimum relative percentage increase of particles in bin ]0.8-1.0] is 25%. Even more preferably, for the circularity and roundness the minimum relative percentage increase of particles in bin ]0.8-1.0] is 30%.

Based on the experiment, when the additive(s) react with the dissolved sulfur, it was concluded that for both the circularity and roundness, the minimum relative percentage increase of the amount of microstructural phases and/or constituent in bin ]0.6-1.0] is typically 10% compared to the microstructural phases and constituents of the powder without the additive. More preferably, the minimum relative percentage increase of the amount of microstructural phases and/or constituent in bin ]0.6-1.0] is typically 15%. Even more preferably, the minimum relative percentage increase of the amount of microstructural phases and/or constituent in bin ]0.6-1.0] is typically 20%.

The experiment illustrates that adding magnesium to a Fe-rich melted metal material before or during a water atomization process, an increase of the sphericity of the atomized powder metal material is obtained, compared to the same material without the added magnesium.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims. 

What is claimed is:
 1. A method of manufacturing a powder metal material, comprising the steps of: adding at least one additive to a melted base metal material, the at least one additive forming a protective gas atmosphere surrounding the melted metal material which has a volume of at least three times greater than the volume of the melted base metal material to be treated; and atomizing the melted metal material after adding at least some of the at least one additive to produce a plurality of particles.
 2. The method of claim 1, wherein the median circularity of the particles is at least 0.60.
 3. The method of claim 1, wherein the median roundness of the particles is at least 0.60.
 4. The method of claim 1 further including the step of heat treating the particles after the atomization step to form microstructural constituents or phases, the microstructural constituents or phases have a median circularity of at least 0.60 and a median roundness of at least 0.60.
 5. The method of claim 4, wherein the metal material is an iron-based material, the at least one additive includes magnesium, the microstructural constituents or phases are graphite precipitates and/or carbides and/or nitrides, and the phases and/or constituents have a median circularity of at least 0.60 and a median roundness of at least 0.60.
 6. The method of claim 1, wherein the atomizing step includes water atomizing, gas atomizing, plasma atomizing, or rotating disk atomizing.
 7. The method of claim 6, wherein the atomizing step includes water atomizing, and the water atomizing step includes applying water at a pressure of 2 MPa to 150 MPa and above to the melted metal material.
 8. The method of claim 1, wherein the base metal material includes at least one of aluminum (Al), copper (Cu), manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe), titanium (Ti), and chromium (Cr); and the base metal material optionally contains at least one alloying element selected from the group consisting of silver (Ag), boron (B), barium (Ba), beryllium (Be), carbon (C), calcium (Ca), cerium (Ce), gallium (Ga), germanium (Ge) potassium (K), lanthanum (La), lithium (Li), magnesium (Mg), molybdenum (Mo), nitrogen (N), sodium (Na), niobium (Nb), phosphorus (P), sulfur (S), scandium (Sc), silicon (Si), tin (Sn), strontium (Sr), tantalum (Ta), vanadium (V), tungsten (W), yttrium (Y), zinc (Zn), and zirconium (Zr).
 9. The method of claim 8, wherein the at least one additive added to the base metal material includes at least one of: K, Na, Zn, Mg, Li, Sr, Ca, and Ba.
 10. The method of claim 8, wherein the base metal material is an iron-based material, and the at least one additive forming the protective gas atmosphere includes at least one of: K, Na, Zn, Mg, Li, Sr, and Ca.
 11. The method of claim 8, wherein the base metal material is iron-based and includes sulfur present as an impurity, and the at least one additive includes at least one of Zn, Mg, Li, Sr, Ca, and Ba to react with the sulfur.
 12. The method of claim 8, wherein the base metal material is iron-based and includes at least one oxide present as an impurity, and the at least one additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca, and Ba to react with the at least one oxide.
 13. The method of claim 8, wherein the base metal material is iron-based and includes sulfur and at least one oxide present as impurities; the at least one additive forming the protective gas atmosphere includes at least one of Zn, Mg, Li, Sr, and Ca; and the at least one additive also reacts with the sulfur and the at least one oxide.
 14. The method of claim 8, wherein the base metal material is an aluminum alloy and includes sulfur and/or at least one oxide present as impurities; the at least one additive forming the protective gas atmosphere includes at least one of: K and Na; and the at least one additive includes at least one of K, Na, Mg, Li, Sr, Ca, and Ba to react with the sulfur, and/or the at least one additive includes at least one of K, Na, Mg, Li, Ca to react with the at least one oxide.
 15. The method of claim 8, wherein the base metal material is titanium-based and includes sulfur and/or at least one oxide present as impurities; and the at least one additive forming the protective gas atmosphere includes at least one of: Zn, Mg, Li, Ca and Ba; and the at least one additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca, and Ba to react with the sulfur, and/or the at least one additive includes at least one of Sr, Ca, and Ba to react with the at least one oxide.
 16. The method of claim 8, wherein the base metal material is a cobalt alloy and includes sulfur and/or at least one oxide present as impurities; the at least one additive forming the protective gas atmosphere includes at least one of: K, Na, Li and Ca; and the at least one additive includes at least one of Na, Mg, Li, Sr, Ca, and Ba to react with the sulfur, and/or the at least one additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca, Ba to react with the at least one oxide.
 17. The method of claim 8, wherein the base metal material is a chromium alloy and includes sulfur and/or at least one oxide present as impurities; the at least one additive forming the protective gas atmosphere includes at least one of: K, Na, Zn, Mg, Li, Sr, Ca and Ba; and the at least one additive includes at least one of K, Na, Zn, Mg, Sr, Ca, and Ba to react with the sulfur, and/or the at least one additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca, and Ba to react with the at least one oxide.
 18. The method of claim 8, wherein the at least one additive includes Mg.
 19. A water atomized powder metal material, comprising: a plurality of atomized particles formed from a base metal material and at least one additive, wherein the atomized particles have a median circularity of least 0.60 and a median roundness of at least 0.60.
 20. The powder metal material of claim 19, wherein the base metal material includes at least one of aluminum (Al), copper (Cu), manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe), titanium (Ti), and chromium (Cr); and the base metal material optionally contains at least one alloying element selected from the group consisting of silver (Ag), boron (B), barium (Ba), beryllium (Be), carbon (C), calcium (Ca), cerium (Ce), gallium (Ga), germanium (Ge) potassium (K), lanthanum (La), lithium (Li), magnesium (Mg), molybdenum (Mo), nitrogen (N), sodium (Na), niobium (Nb), phosphorus (P), sulfur (S), scandium (Sc), silicon (Si), tin (Sn), strontium (Sr), tantalum (Ta), vanadium (V), tungsten (W), yttrium (Y), zinc (Zn), and zirconium (Zr).
 21. The method of claim 20, wherein the at least one additive includes at least one of: K, Na, Zn, Mg, Li, Sr, Ca, and Ba.
 22. A water atomized powder metal material, comprising: a plurality of atomized particles formed from a base metal material and at least one additive, wherein the atomized particles include microstructural constituents or phases, and at microstructural constituents or phases have a median circularity of at least 0.60 and a median roundness of at least 0.60.
 23. The powder metal material of claim 22, wherein the base metal material includes at least one of aluminum (Al), copper (Cu), manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe), titanium (Ti), and chromium (Cr); and the base metal material optionally contains at least one alloying element selected from the group consisting of silver (Ag), boron (B), barium (Ba), beryllium (Be), carbon (C), calcium (Ca), cerium (Ce), gallium (Ga), germanium (Ge) potassium (K), lanthanum (La), lithium (Li), magnesium (Mg), molybdenum (Mo), nitrogen (N), sodium (Na), niobium (Nb), phosphorus (P), sulfur (S), scandium (Sc), silicon (Si), tin (Sn), strontium (Sr), tantalum (Ta), vanadium (V), tungsten (W), yttrium (Y), zinc (Zn), and zirconium (Zr).
 24. The method of claim 23, wherein the at least one additive includes at least one of: K, Na, Zn, Mg, Li, Sr, Ca, and Ba.
 25. The powder metal material of claim 23, wherein the base metal material is an iron-based material, the at least one additive includes Mg, the microstructural constituents or phases are graphite precipitates, and the graphite precipitates have a have a median circularity of 0.60 and a median roundness of 0.60. 